Building a single-family house with a basement - Is the financing calculation too risky?

I unfortunately do not understand the first sentences,

2008 and 2009 as well? I know some salespeople from various industries who got massively into trouble

4 m on the size is not a steep slope, there are other plots for that. We have 13 m

I would integrate the basement as a living floor. A granny flat for the children maybe later sounds a) like an emergency solution
b) lack of imagination from the planner
Basement is either granny flat or storage room

Regarding the costs
30,000 additional construction costs are too little especially for a slope location. They might be enough for a flat site.
110,000 for the basement, what is included there? For earthworks, waterproof shell, and finishing to living quality it is too little
100,000 for the outdoor area considering the plot size, slope location, presumably also rather upscale wishes, without own work I consider that very tight. I would already look for a garden landscaper and talk to him.

For me, the debt would be too high with that income. However, losing a job or long-term illness doesn’t take the air from life. Rethink the granny flat

Great, then the only remaining risks are that the kids consume more than is good for them and the risk of unemployment.
If this setback can be bridged over a period of time, then everything is fine.
I would still be careful if I were you. Few people make extra repayments because the money is misused for other things.
The wishes you have are quite extravagant, where the brand matters more than the item itself. That then also rubs off on the children, who then go through 3 iPhones a year at the age of 12.
But you'll see that then